(a) Field of the Invention
The subject invention relates to a marine exploration system and more particularly, but not by way of limitation, to a marine exploration system using a ship with a towed “fish”. The fish used for measuring telluric currents as a natural electromagnetic energy source flowing beneath an ocean or sea floor.
(b) Discussion of the Prior Art
Heretofore, ground and airborne electromagnetic systems have been in use for natural resource exploration from about 1950 onwards. These systems depend mainly upon the measurement of the magnetic and conductive properties of the underlying ground. Airborne magnetic survey systems, that employ magnetometers with advanced stages of development, provide very satisfactory results. However, airborne conductivity measurements of the underlying terrain made with airborne electromagnetic systems that currently exist, leave a great deal of room for improvement.
Electromagnetic systems typically operate at a minimum terrain clearance with respect to safety and employ electromagnetic transmitters operating in the frequency range from about 20 Hz. to 50 kHz with limited ground penetration. The ground currents and their related secondary magnetic fields as induced in the underlying ground by these transmissions are detected by receiving coils mounted in a tail boom on an aircraft or in an airborne survey bird towed behind the aircraft. Either fixed wing aircraft or helicopters are used for these surveys. The response from the underlying ground is related to it's conductivity and the depth of penetration of the transmitted fields. The latter is primarily a function of the frequency employed and the field strength of the electromagnetic field that is generated by the equipment. Typical maximum penetrations are in a range of 400 to 1000 ft.
The only exception to the above description was an airborne system known as “AFMAG” that was developed by S. H. Ward and others in the 1960's. (S. H. Ward et al. AFMAG-Applications and Limitations. Geophysics, Vol. XXXI, No. 3 (June 1966), pp. 576–605.) This system utilized the natural electromagnetic fields generated by lightning events occurring in distant electrical storms. These storms can provide a source for electromagnetic energizing of the ground, primarily in the frequency range of 20 Hz. to 500 Hz. Useable frequencies down to about 3 Hz. exist but high quality receiving coils and coil anti-vibration mountings are required for the lower frequencies. These were apparently not available in the AFMAG system.
Although the AFMAG system showed some promise, it did not achieve sufficient commercial acceptance to survive for more than a short period. Amongst the various problems of the system was the absence of the sophisticated instrumentation and digital data acquisition and processing systems that were not available at that time. Also and very importantly, there was a lack of adequate technology for suppressing the prime sources of noise, such as angular vibration of the detection coils in the presence of a strong magnetic field in the earth. The latter is associated with a motor generator effect that can detect a millionth of a degree of angular vibration.
The AFMAG system was also restricted to the use of audio frequency fields and did not employ extremely low frequency and much more powerful natural magnetotelluric fields, as used in the present invention. Just as importantly, the AFMAG system as well as all other airborne electromagnetic systems, past or present, did not make use of the valuable data available in the electric field components of electromagnetic fields.
The subject marine exploration system demonstrates that electric field data, as measured by methods that do not make contact with an ocean or sea floor, can be more important than the magnetic component of electromagnetic fields. Experience with the invention has also shown that, for specific reasons, the measuring of the electric field data is particularly valuable at frequencies below 3 Hz. This type of information is completely missing in the old AFMAG system as well as current marine, ground and airborne electromagnetic systems. The field data lies in the range of frequencies from 0.01 Hz. to 3 Hz. and is used in the present invention for the marine detection of an induced polarization phenomena. Also, related response of dielectric interfacial polarization effects can be detected over oil and gas fields. These low frequency polarization effects, which are particularly important in the electric fields, are discussed herein.
In U.S. Pat. No. 6,765,383 issued to the subject inventor, a magnetotelluric geophysical survey system is described using an aircraft survey bird. The survey system uses natural electromagnetic EM fields as an energy source. The system includes the survey bird with electric dipoles, an angular motion detector and an airborne data recording system. The subject marine exploration survey system described herein is similar to the survey system described in U.S. Pat. No. 6,765,383 and provides a unique marine exploration system using a combination of non-contact electric field EM techniques for oil and gas exploration under an ocean and sea floor.
The marine exploration system is dependent upon certain effects associated with a deep flow of natural telluric currents in a frequency range of 0.01 to 3 Hz. These currents are induced in the earth by an action of magnetotelluric (MT) fields. These MT fields are well known and are generated by the interaction of solar wind with the outer reaches of the earth's magnetic field. The solar wind is a stream of positively charged particles that are emitted by the sun. The MT fields that are relevant to the subject exploration system cover a frequency range from approximately 30 Hz down to 0.01 Hz and lower. These MT fields increase progressively in field strength with decreasing frequency, which is a fact that provides for penetrations into an ocean and sea floor at frequencies of 1 Hz. and lower down to depths of 10,000 feet and deeper.